Pressurized water nuclear reactors incorporate a primary circuit in which water circulates at a pressure close to 155 bars and a temperature close to 300.degree. C. for cooling the reactor core and transferring the heat produced by this core to the steam generator. In the primary circuit of the reactor the pressurized water circulates at a very high flow velocity and its corrosive power is great, since this water contains boric acid employed to modify the reactivity of the reactor core.
During the operation of the reactor, the pressurized water at a high temperature and with a high corrosive power produces particles of oxide (of iron, nickel or cobalt), during its contact with the components of the primary circuit, and entrains these particles at a very high velocity. An appreciable proportion of these particles, consisting mainly of oxide, is in colloidal form, increasing the difficulty of the operations for removing these particles.
Moreover, nuclear reactors operate continuously and are usually stopped only annually, for operations of maintenance and reloading of the reactor fuel. Over such a long period, the quantity of particles present in the primary water would reach too high a level and it is necessary to remove these particles during the operation of the reactor, in order to reduce the radioactivity of the primary fluid. In fact, through the action of radiation, during their passage through the core fuel assemblies, the oxide particles give rise to highly radioactive isotopes. The harmful nature of these phenomena is further increased by the fact that the radioactive corrosion products can accumulate at certain particular places in the circuit, creating sites of high radioactivity.
The upper limit of the content of solid constituents in suspension in the primary water has therefore been fixed at a value of the order of 1 ppm (10.sup.-6).